This application claims the priority benefit of Taiwan application serial no. 90122077, filed Sep. 6, 2001.
1. Field of Invention
The present invention relates to a piezoelectric printhead and its method of manufacture. More particularly, the present invention relates to a piezoelectric printhead that uses a metallic layer and a thick film layer with a slot hole therein instead of conventional ceramic material to form a vibration layer and an ink cavity layer structure.
2. Description of Related Art
In general, the operating mechanism of a conventional ink-jet printer can be classified into thermal bubble and piezoelectric. Thermal bubble ink-jet printing utilizes a heater to vaporize an ink drop quickly to form a high-pressure gaseous ink bubble so that the ink is suddenly ejected from an ink nozzle. Because thermal bubble print head is inexpensive to produce, they are mass-produced by commercial companies such as HP and Canon. However, the high-temperature vaporization mechanism needed to operate the printhead often limits the type of ink (mainly a water-soluble agent) that can be selected. Such limitations narrow its field of applications.
Piezoelectric printing utilizes the deformation of a block of piezoelectric ceramic material when a voltage is applied. Such deformation compresses liquid ink and creates a liquid jet out from an ink reservoir. Compared with a thermal bubble type of print head, a piezoelectric printhead has several advantages. Unlike a thermal bubble printhead that demands the ink to be vaporized at a high temperature and hence may change the color somewhat, the piezoelectric printhead has no such problem. Furthermore, the piezoelectric printhead operates without cyclic heating and cooling and hence may have a longer working life. Moreover, the piezoelectric ceramic material responds to a voltage quickly and hence may produce print documents a lot faster. The response of a thermal bubble printhead, on the other hand, is limited by the rapidity of heat conduction. Last but not least, the amount of deformation in the piezoelectric ceramic depends on the voltage of the electricity applied. In other words, by controlling the voltage applied to the piezoelectric ceramic, size of the ink droplet ejected from a nozzle may change. Ultimately, quality of the document produced by the piezoelectric printhead can be improved.
FIG. 1 is a schematic cross-sectional view of a conventional piezoelectric ink-jet print head. Ceramic green tapes for forming a conventional piezoelectric ink-jet print head 100 including an upper electrode layer 102, a piezoelectric layer 104, a lower electrode layer 106, a vibrating layer 108, an ink cavity layer 110 and an ink cavity bottom film layer 112 are manufactured in thick film processes. Thereafter, the green tapes are pressed together in the correct order and fired to form a ceramic structure such as the piezoelectric ink-jet printhead manufactured by EPSON.
To operate the piezoelectric printhead 100, a voltage is applied to the piezoelectric layer 104 through the upper electrode 102 and the lower electrode 106. Since the piezoelectric layer 104 is a piezoelectric ceramic material, the piezoelectric layer 104 will deform pushing the vibrating layer 108 and pressuring the ink inside the ink cavity 114. A portion of the pressurized ink ejects from an ink nozzle 116 and travels to a paper document to form a dot pattern.
In a conventional piezoelectric ink-jet printhead, aside from the metallic upper electrode and the lower electrode, other layers are separately formed in thick film ceramic processes and then combined together by pressure and high-temperature treatment. Consequently, a conventional piezoelectric ink-jet printhead has the following disadvantages:
1. Since the piezoelectric ink-jet printhead has a relatively small dimension but a relatively high precision, various thick ceramic films must be carefully aligned before being joined together. This may lead to a lowering of product yield.
2. Because the piezoelectric printhead has a relatively complicated structure, the ceramic material may shrink unevenly during a thermal treatment process leading to stress or structural damage. Again, this may lead to a drop in product yield.
3. The uneven shrinkage due to a high temperature treatment may also lead to a mismatch between delicate parts within the ink-jet printhead. This aspect of the production not only lowers product yield, but also decreases the packing density of ink-jet printheads leading to a lower print resolution.
Accordingly, one object of the present invention is to provide a method of forming a piezoelectric ink-jet printhead. The method uses an electroplating process to form a metallic layer instead of using ceramic material to form a vibration layer and uses film forming (roller coating), exposure and developing processes (photolithography) to form a thick film layer instead of using ceramic material to form an ink cavity layer. Hence, product yield and manufacturing precision are increased.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a piezoelectric ink-jet printhead. The piezoelectric printhead has a substrate with a metallic layer thereon. A lower electrode layer is formed over the metallic layer. A patterned piezoelectric layer is formed over the lower electrode layer. A patterned upper electrode layer is formed over the piezoelectric layer. A patterned thick film layer is formed over the metallic layer. The thick film layer includes at least a slot hole that passes through the thick film layer. The thick film layer and the metallic layer together form a cavity. The cavity encloses the upper electrode layer and the piezoelectric layer. A nozzle plate is formed over the thick film layer. The nozzle plate, the thick film layer and the metallic layer together form an ink cavity. The nozzle plate further includes a nozzle hole linked to the ink cavity. The piezoelectric ink-jet printhead further includes an inert layer between the lower electrode layer and the metallic layer. The inert layer is made from an inert metal or an insulating material.
This invention also provides a method of forming a piezoelectric ink-jet printhead. A substrate having a first and a second surface is provided. A metallic layer and a lower electrode layer are sequentially formed over the first surface of the substrate by electroplating. Thereafter, a patterned piezoelectric layer and an upper electrode layer are sequentially formed over the lower electrode layer by screen-printing. A patterned thick film layer is formed over the metallic layer by film forming (roller coating) and an exposure/development process. The thick film layer has at least a slot hole that passes through the thick film layer. The thick film layer and the metallic layer together form a cavity. The cavity encloses the upper electrode layer and the piezoelectric layer. A nozzle plate is attached to the thick film layer. The nozzle plate, the thick film layer and the metallic layer together form an ink cavity. The nozzle plate has a nozzle hole continuous with the ink cavity. After forming the metallic layer, an inert layer may also be formed over the metallic layer. The inert layer is made from an inert metal or an insulating material. In addition, a firing process may be performed after forming the piezoelectric layer.
In this invention, a metallic layer formed by electroplating replaces the conventional ceramic vibration layer. Since electroplating costs less than forming a ceramic thick film by compression, production cost of the print head is reduced.
This invention also uses exposure/development processes to form a slot hole in the thick film layer. The slot hole and the metallic layer together form a cavity and the thick film layer with a slot hole therein serves as an ink cavity layer for the ink-jet printhead. Because exposure/development processes are capable of producing a pattern with great accuracy, dimensions of the ink cavity can be precisely fabricated.
In addition, the piezoelectric layer and the upper electrode layer are enclosed within the ink cavity instead of outside the cavity so that overall thickness of the ink-jet printhead is reduced. Hence, there is a volume reduction of the ink-jet printhead.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.